The present invention relates in general to color electrooptic displays and, more particularly, to variable color displays which utilize electrooptic materials together with polarizers and a retardation plate in order to achieve an optimum combined effect of high contrast and brightness, wide viewing angle, and relatively low operating voltage requirements. More specifically, the invention is directed to a color display in which the color of the displayed indicia can be selected to be one of two or more predetermined colors, and is further directed to a color display wherein the color of the displayed indicia and of the background can be continuously varied.
Ferroelectric ceramic materials, such as PLZT, are optically transparent and have electrically-controllable light modulation properties. In these ferroelectric ceramic materials, light modulation may be produced by two modes: (1) light scattering, which has not proven to be desirable for displays because the achievable contrast ratios are too low, and (2) optical birefringence, also known as the electrooptic effect, which is a material phenomenon wherein very high contrast ratios on the order of 5,000 to 1 are attainable.
In evaluating these approaches, it is helpful to distinguish between the materials utilized; for instance, whether the material utilized is a single crystal structure or a ceramic material. This is important because single crystal materials are different from the ceramic materials in that ceramic materials are composed of random aggregates of microscopic crystallites, on the order of 1 to 15 micrometers in average diameter, intimately bonded or sintered together to form a dense solid material. Light scattering effects are minimal to non-existent in small grain-sized materials (less than 2 microns) whether the material used is of the memory type or the non-memory type. However, birefringence is observable in both large grain size (greater than 2 microns) non-memory materials, and small grain size non-memory materials; but birefringence in ceramic memory materials is useful only in small grain size types. Optical transparency is enhanced when large grain size ceramic material is utilized.
It is known that conductive electrodes can be disposed on ceramic material to induce an electric field. The electrical field vector, which is determined by the placement of the electrodes and the polarity of the applied voltage, defines the internal polarization direction of the ceramic material, i.e., the atomic unit cell elongation.
The PLZT ceramics are optically birefringent, uniaxial materials, which are substantially transparent in the wavelength region from 0.37 to 6.5 micrometers. The PLZT plates are defined as uniaxial because they possess one unique direction, i.e., the polarization direction, along which light travels at a different velocity relative to the other two orthogonal directions. It is important to recognize that PLZT ceramics possess optically uniaxial properties on a microscopic scale and also on a macroscopic scale when polarized with an electrical field. In uniaxial crystals, there is one unique symmetry axis, the optic axis, which is colinear with the ferroelectric polarization vector in the PLZT ceramics and which possesses different optical properties than the other two orthogonal axes. That is, light traveling in a direction along the optic axis, and vibrating in a direction perpendicular to the optic axis, encounters a different index of refraction than the light traveling in a direction at right angles to the optic axis and vibrating parallel to the optic axis. The difference in velocities, or indices of refraction, is known as the birefringence or .DELTA.n, (where n=c/v where c=velocity of light in a vacuum, and v is the velocity of light along a given crystalline direction). Stated another way, the absolute difference between the two indices is defined as the birefringence, i.e., n.sub.E -n.sub.O =.DELTA.n. The inherent optical activity and ability to cause optical retardation or phase delay is proportional to the .DELTA.n of the material. On a macroscopic scale, .DELTA.n is equal to zero before electrical poling and has some finite value after electrical poling, depending on the composition of the ceramic material utilized and the degree of polarization.
The .DELTA.n value is a meaningful quantity in that it is related to the optical phase retardation in the ceramic material. For certain compositions within the PLZT materials, i.e., ferroelectric non-memory type ceramic material such as 9/65/35 (9% La, 65% PbZrO.sub.3 and 35% PbTiO.sub.3), .DELTA.n is electrically induced and is proportional to the square of the electrical field strength. This results in a quadratic ceramic material, since .DELTA.n=kE.sup.2. The subject matter of the present patent application preferably utilizes such ceramic materials.
These ferroelectric ceramic materials, by virtue of their natural cubic symmetry, do not possess permanent polarization and are not optically birefringent in their quiescent state. Such PLZT ceramic materials contribute no optical retardization to an incoming light beam. However, when an electrical field is applied to the PLZT ceramic materials, electrical polarization and birefringence is induced in the ceramic materials, and optical retardation occurs. A linearly polarized light, on entering the electrically energized ceramic material, is resolved into two perpendicular components, whose vibration directions are defined by the crystallographic axes of the crystallites acting as one optical entity. Because of the different refractive indices, n.sub.E and n.sub.O (i.e., the respective index along the propagation direction and the respective index perpendicular to the propagation direction), the propagation velocity of the two components will be different within the ceramic material and will result in a phase shift called retardation. The total retardation .GAMMA. is a function of both .DELTA.n and the optical path length t (i.e., the thickness of the ceramic PLZT plate), according to the relation .GAMMA.=.DELTA.n.times.t. When sufficient voltage is applied to the PLZT ceramic material, a half-wave retardation is achieved for one component relative to the other. This results in the vibration direction of the linearly polarized light being rotated by 90 degrees. Switching from the state of zero retardation to half-wave retardation will create a light shutter when the plate is placed between "crossed" polarizers, i.e. polarizers having orthogonal polarization axes.
Displays have been used in a variety of applications including the visual presentation of information in vehicles. For example, vehicular "head-up" display systems have been disclosed wherein the desired information is displayed in the line of sight of the person controlling the vehicle which may comprise an automobile or airplane. Such systems suffer from a multiple image reflection problem in which the observer sees one or more secondary images of the displayed information which is offset from the primary desired image. The secondary images result from refractions in the viewing screen which may comprise the windshield in an automobile.